The biggest problem is that a TC magnifies lens flaws, and the 120-300 has plenty of them when wide open.A 100-400mmL will be noticably sharper at 400mm. If you must have f/4, try and find a older Canon 300mm f/2.8. Even with a 1.4X TC, its very sharp.

I recently bought a used 300mmf4L Canon which is sharp and light. $700. I tried a friend's Canon 100-400L and it's nice to have the flexibility and the extra 100mm past 300, but for my purposes (people) I will be too far away with 400 to direct what is happening. the f4 at 300 is nice and sharper than I thought it would be.

Yes, an old lens design. Focus is OK though. Has IS.

Can you live without zoom? I sometimes reach for the non-existant zoom ring...(I have a 70-200)

Has the 2012 revision of the Sigma 120-300 f/2.8os actually started shipping yet? It's a lens I've had an eye on for years, but never felt quite confident enough to buy, especially as I already have a stellar copy of the EF300 f/2.8is. But wouldn't the flexibility of the Sigma's 120-300mm range be brilliant if all the elements worked as promised?

Something I've read on multiple occasions about the 120-300 is the contention that it's not a true 300mm at the long end....more like 280mm. Just something else to be aware of if you find yourself pulling out the Visa card...

The other thing to consider is all the evidence that Sigma has really sharpened up their whole operation with some stunning new releases in the past few months and evidence that the once highly variable QC issues may soon become a distant bad memory.

I'd be waiting to read genuine reviews of the 2012 revision of the Sigma 120-300 f/2.8os and if it stacks up, then drop the Visa card on it.

I have owned the 120-400 OS and the 120-300 2.8 non-OS. The 120-400 sucks compared to the 120-300 by a huge margin. IQ, AF lock and AF tracking on the 120-300 kick butt, even with IQ the only thing really suffering with a TC. I have used a 120-300 with 2x many times and the contrast and sharpness do take a hit but it with a 1.4x there is not a huge difference even with the Canon 1.4x mkI. The 2012 120-300 has OS, better IQ and weather sealing...worth every cent!

I want to rent the NEW 120-300 f/2.8, which will include two fluorite elements. (nobody knows when it will get released). But I still will not be able to buy one anytime soon.

So I'm thinking of buying the 120-400. The pictures I have seen online, look plenty sharp to me.

If you were using a Rebel body, it may be the camera that is more at fault for the low light AF problems. If you were using a 60D, then that would be of more concern to me, because my 50D has a similar AF...but mine has AFMA.

I have only seen one negative review on amazon, I think from 2010, that said they couldn't get the AF dialed in using AFMA. The rest of the reviews seem pretty positive...like 50 reviews and it's at like 95% approval. That's about as good as reviews get.

I've been using the sigma 120 400 on a 7d, 60d and 5d mk iii. Using it on the 60d did show a heavy front focus (+15) which I could correct on 7d and 5d.

It lacks contrast and is a little soft @ 5.6 but gets plenty sharp @ 6.3-7.1 and above.

I find AF on all bodies to be OK in good light, but not great. trying to track anything moving in less than good light (i.e. under woodland cover shade), gets tricky. The AF is better on the 5d but it still hunts a bit in low light.

It was the first lens I bought after I got my 7d as I couldn't afford a big tele at the time. I've been quite happy with it in general and to be honest was a bargain @ £600 I got it for back then. But now I'm after something with a wider aperture, but hopefully with similar reach.

Alex, thanks very much for the info! In the low light conditions and woodland shade, etc...was that in servo AF mode? And how many AF points were selected on the various bodies?

Would you care to post a cropped version of one of your sharpest shots with it? If you have already, I apologize for missing it.

When you did the AFMA, did that remain the same throughout the zoom range, or was the adjustment only good for the long end, etc? That would be my biggest concern.

Oh, and at what point does it become f/5.6? At 200mm, 250mm, 300mm? Hopefully not below 200mm...

As for similar sized lenses, I've only rented the Canon 400mm f/5.6 prime, and the 300mm f/4. (I've rented a couple of bigger superteles also). I found the 400 to AF extremely well in very low light, 10 to 20 minutes past sunset (depending on if there were diffuse clouds, or if the sky was clear). I was using center point only, in single shot (not servo) AF mode. Of course to get decent shutter speed, I was too high into the ISO, but it was fun to try anyway.

I found the 400's sharpness to be extreme, and contrast very good. Closing it down up to about f/7.1 only helped a little, and actually began to reduce what I would call global contrast...as most all lenses do as you close the aperture (sometimes that's an advantage though). Using the Canon 2.0x ii TC, looked a bit better on the 400, than on the 300. Of course I had no autofocus with it connected to the 400 on my camera, but it did work on the 300 f/4, mounted to a 1D Mark 4. I had no 1.4x TC on hand to try, but I have no doubt it should work superbly on the Canon 400 f/5.6, especially on a full frame camera...provided you could make it autofocus.

Obviously the faster superteles will go sharper and have better color and contrast, will provide a less noisy image with better bokeh, and will AF much better...but you do pay a lot for all that, and they're bulky.

Check out the full size "original" versions. Of course those cameras' pixels are huge compared to my crop camera...but the sharpness of the float plane pictures looks fully professional to me. They could almost be used in some kind of ad campaign. Makes me wonder if he used much AFMA, if any...because the focus he got is always "tack sharp" regardless of the focal length he used. Of course most were shot at f/8, which no doubt helped a bit.

I kind of hate that I still envy pretty much all pictures I have ever seen, regardless of lens, shot with a D700. Yes, the 12MP pixel dimensions of the image are a lot smaller than a 5D2 or 3, but the ultimate resolution of details, surely isn't that far behind, if at all (especially at higher ISO). But...it's Nikon... !!

The reviews of the Sigma lens say the color is "warmer" than the Canon 100-400. But that is exactly what I want. The Darwin Wiggett review, and images like the above, are what has convinced me to try the lens. The photozone.de review, looks like the lens is positively horrible. Based on the above pics, it looks like they got a bad sample, and Jeff got a perfect sample.

Some of the other shots on pbase, done with an older Canon Rebel and the Sigma 120-400, look unimpressive, obviously the lack of AFMA is a big factor, as well as the wimpy AF of those cameras in the first place. The bokeh, especially background bokeh, looked kind of bad on those as well...but that is something I need to try for myself, to see if I can live with it. Background bokeh with a long telephoto, even a Canon L, isn't always the smoothest anyway. Even the 200L I tried, had background bokeh that did some unusual things...of course if the background is pushed enough out of focus, then it looked very smooth and pleasing...so those anomalies don't show up.

I'm hanging out for the revision model of the 120-300. The new Sigma 120-300mm f/2.8 DG OS HSM lens will be available in Canon, Nikon and Sigma mounts. Expected ship date is March 16, 2013. Not long now!

PW, thank you, that's good to know! I look forward to trying it the new 120-300, but it will be a long time before I can buy. I need to get a second job as an exotic car parker, or gigalo or something!

In the low light conditions and woodland shade, etc...was that in servo AF mode? And how many AF points were selected on the various bodies?

Would you care to post a cropped version of one of your sharpest shots with it? If you have already, I apologize for missing it.

When you did the AFMA, did that remain the same throughout the zoom range, or was the adjustment only good for the long end, etc? That would be my biggest concern.

Oh, and at what point does it become f/5.6? At 200mm, 250mm, 300mm? Hopefully not below 200mm...

I used it with 7d, 60d and 5dmk iii. I only ever adjusted the tele end AFMA as the wide end was quite sharp. think it becomes 5.6 @ 250.

I've used it with a variety of AF points, single shot, servo etc.. and generally found the AF performance to be mediokre in less than good light. (probably my own fault for buying a canon70_200 IS 2.8 II and trying to compare the two )